It has been included in the base install since RHEL 5.1. If it's not on your RHEL system, run
yum install yum-securityTo list available security errata, run
yum --security check-updateTo install only security-related errata, only up to the latest security errata package, patch using the following command:
yum --security update-minimalPatching with precision should result in more secure systems with more uptime and more stability. See the yum-security man page for details and examples.